Re: [PATCH v2] keys: Fix UAF in key_put()

[David Howells]

Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:

> > +		if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) {
> > +			smp_mb(); /* Clobber key->user after FINAL_PUT seen. */
> 
> test_bit() is already atomic.

Atomiticity doesn't apply to test_bit() - it only matters when it does two (or
more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW).

But atomiticity isn't the issue here, hence the barrier.  You need to be
looking at memory-barriers.txt, not atomic_bitops.txt.

We have two things to correctly order and set_bit() does not imply sufficient
barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment
about really wanting a set_bit_release().

> > +			smp_mb(); /* key->user before FINAL_PUT set. */
> > +			set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> 
> Ditto.

Ditto. ;-)

> Nit: I'm just thinking should the name imply more like that "now
> key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE
> would be more self-descriptive.

KEY_FLAG_PUT_DONE isn't right.  There can be lots of puts on a single key -
only the one that reduces it to 0 matters for this.  You could call it
KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE.

David